Storage mode operation of a photosensor



Feb. 11, 1959 G, p, WECKLER l' 3,427,461

STORAGE MODE OPERATION OF A PHOTOSEN-SOR Filed Feb. 23, 1966 INV ENTOR.

ATTQRNEYS United States Patent Oice Patented F eb'. 11, 1969 8 ClaimsABSTRACT OF THE DISCLOSURE A photosensor device for sensing radiantenergy including a transistor and an energizing means coupled to thetransistor for periodically charging the space-charge capacitance of oneof the transistor junctions, the charged capacitance remaining after theremoval of the energizing means to prevent leakage currents from flowingfrom the one junction through the other junction during the inter veningperiods when the one junction is not being charged.

This invention relates to a photosensor device and a method of operatingsuch a device.

Solid-state elements, such as diodes and transistors, have lbeencommonly employed as photosensors. Recently, it was realized that thereis a signicant advantage in operating a diode in `what is referred to asthe storage mode of operation. The storage mode involves reverse biasing,the p-n junction of a diode to charge the associated depletion layercapacitance. The diode is then open circuited and the capacitancedischarges at a rate proportional to the radiant power impinging on thep-n junction. The charge required to restore the voltage across thejunction is a measure of the impinging radiant energy. Thus, in thestorage mode of operation, the radiant power impinging upon theIjunction is integrated for a period To (which is referred to as thescan time). The time :between the commencement of repeated charging ofthe diode junction is referred to as the repetition period Tp. The timeperiod in which the junction is charged is referred to as the sampletime At. The ratio T l,/Az is the approximate gain obtained in thestorage mode of operation. From this brief explanation, it can be seenthat the advantages of the storage mode of operation are: (1) an outputsignal having a linear dependence on the impinging radiant energy; (2) asubstantial gain in the output signal as com.- pared with a simple p-njunction photosensor; (3) the ability to detect several orders ofmagnitude of radiant energy by adjusting the sample and scan timeswithout loss of other desirable characteristics; and (4) a compatabilitywith sequential or random accessing of one and two dimensional arrays ofelements.

The practical use of the storage mode of operation has only recentlybeen achieved. One obstacle to this has been the lack of practicalswitches to open circuit the diode during the discharge of thespace-charge capacitance in a manner that leakage currents do notsignificantly discharge the space-charge capacitance and substantiallyaffect the measure of incident radiation. The problem is compounded bythe additional requirements that the switch must operate at high speedand provide a lowimpedance charging path. It is also desirable that theswitch be compatible with semi-conductor large array fabrication. Oneprior art switching system shown in U.S. Patent No. 3,011,089 employs anelectron-beam gun to switch an array of diodes. The use of a relativelycomplex electron-beam gun as a switch decidedly limits the applicationof this device. Another arrangement as shown in U.S. patent applicationSer. No 434,916, led Feb. 24,

1965, by Louis I. Kabel1 and assigned to the assignee of thisapplication, overcomes many of the prior art problems iby employing ametal-oXide-silicon transistor (MOST) and diode in combination.

This invention overcomes the shortcomings of prior art devices in asimple, practical and effective manner. Briefly, the structure of theinvention comprises a transistor and an energizing means coupled to thetransistor for repeatedly charging the space-charge capacitance of onejunction of the transistor and for preventing leakage currents from saidone junction via the other junction when the one junction is not beingcharged.

Briefly, the method of the invention for sensing radiant energycomprises impinging radiant energy on a transistor and operating saidtransistor in a storage mode. The use of a transistor enhances theoutput signal in proportion to the beta of the transistor, provides alow-impedance charging path, and a very high-impedance discharge path.The structure may be readily integrated into a single body ofsemi-conductor material by employing Well-known photoengraving anddiffusion techniques.

The above generally-described structure method and advantages, alongywith other advantages of this invention,

are described in the detailed description which follows and theaccompanying drawings, wherein:

FIG. 1 is an electrical schematic circuit diagram of the inventedphotosensor;

FIG. 2 is a partial electrical schematic of the photosensor device shownin FIG. 1 with a sample pulse supplied thereto; and,

FIG. 3 is a partial electrical schematic of the photosensor device shownin FIG. 1 with the pulse terminated (during scan time).

lReferring to FIG. 1, the invented photosensor comprises a transistor10, lan cnerization means f16 and :an output means '24. The Itransistori10 is show-n las an NPN transistor but a 4PNP transistor would operateas well provided the necessary polarity changes were mad-e inenergization means I16 and output means 24. i'lransistor `10 has anemitter 9 'and =a base 1\1 forming a lirst junction 12 (e.g.,emitter-base junction) and a collector 13 which along with base 1'1vform a seco-nd junction '14 I(e.g., collector-base junction). Thecapacitances CEB and CBC associated with the emitter-base junction andthe collector-base junction, respectively, are shown external to Ithetransistor structure as lbroken lines, in Aorder to simplify thedescription of rthe Ioperation of the invented 'device which lappearslater in the specicalt-ion. The `transistor l10 preferably takes theliorm of la double-:diffused planar transistor. 4In such a transistorwith both junctions |12 and `1'4 reversed biased, the DC impedance ofemitter-base junction |12 will be much greater than that of theibase-collector junction 14. Stated Ianother way, since the generationrecombination current of a reverse- Ibiased junction depends directlyupon fthe junction area land :directly upon the space-charge width, itis possible in a double-diffused structure to make the leakage currentof the reverse-'biased emitter-base junction much less than that of thereverse-biased collector-base junction. This is *because the emitterlare-a is always much less than the collector larea in such -a structureand, secondly, because Ithe space-charge width ci an emitter-basejunction in a double-diffused structure is also much less than that of abase-collector junction when at the same voltage. The emitterJbasejunction, therefore, isolates the lbase-collector junction from the restof the circuit during the scan time T0 (FIG. l). Typically, the ratio ofemitter-base to collector-base leakage current would Ibe approximately12100 (i.e., -tw-o orders of magnitude difference). ln general, theare-a of the collector-base junction is at least twice that of theemitter-base area.

Energizat-ion means l'16 is coupled 'to transistor 10 and tunctionsrepeatedly to charge the space-charge capacitance CBC of junction 14 andto prevent leakage from the charged junction 14 via junction 12 whenjunction 14 is not being charged. The energization means -16 comprisespulse means '18, matching resistor 20, and lo-ad resistor 22. Therepeated charging of junction 14 is accomplished by pulse means 11Swhich -repeatedly and periodically supplies a pulse -to the emitter oftransistor to forward bias junction i2. yIn the case of an NPNt-ransistor, pulse means -1l8 supplies la Inegative polarity pulsehaving an amplitude --Vo to the emitter of transistor 10 (FIG. 1), whilein the case of a "PNP, a positive polarity pulse is supplied. The pulsemeans 18 lmay take the lform of any of the |well-known circuits employedto provide a periodic pulse of Iamplitude -V0; period To-j-At; fandpulse Iwidth At, as shown in iFlG. 1. The time At is determined by thevalue of the Ibase-collector junction capacitance CBC, the value ofresistor 22, and the rcomi-non emitter gain -of the transistorstructure, as is known in the ar-t. lt is sometimes desirable yfor pulsewidth Af to be long enough to allow the tnansient condition created bythe energization with a pulse amplitude -Vo to 'die away. Typically, thepulse associated with the time At is in the micro-second range and hasWell-defined leading :and ltrailing edges. The period To-j-At isdetermined lby the range of illumination levels v one wishes to detect,the generation-recombination current, and the value of the voltage Vo.The maximum Kvalue of the 'voltage V0 is determined by the breakdownvoltage of the emitter-base junction l12. .fThe pulse width At and thepulse amplitude V0 should be relatively constant (PFIG. 2).

The remaining portion of energ-ization means l16 is [formed by Ianimpedance network comprising Iresistors and 22. The network 20 and 22cooperates with junction 12 to prevent leakage tfrom the chargedjunction 14 via junction 12 when junction 14 is not lbeing charged bythe pulse-s supplied by pulse means 18. When the pulse from pulse means`118 is termin-ated, junctions 112 and '14 :are reverse bia-sed.Resistor `20, which provides a DC return path around the circuit loop,may be considered as part of pulse means 11S, or as a terminatingresistor across terminals 1-41. lThe value of resistor 20 depends on theparticular device and the particular application. Resistor l22 serves asa load resistor across terminals 2-2 `for the purpose of signalrecovery. It is desirable to have the ratio of resistor 22 to resistor20 large since the voltage amplitude of the output signal `depends uponthis ratio. The value of resistor `22 is restricted by the speed ofoperation desired as it in part determines the speed of operation `whichmay be ach-ieved.

'An output means 24 is coupled across resistor 22 and functions tomanifest an ette-ct representative of the charge supplied -to junctionl14 of transistor .10 during the supply of pulses to the emitter ofitransistor 110. The output means may `take the form of a device, suchas an oscilloscope, providing a visual indication or it may be somecircuit that forms part of a video system or a circuit that ilorrns partof a computer. Thus, output means 24 may ltake the fform of any of thewell-known signal ldis play or utilization devices.

The operation of the photosensor device will now be considered withreference to FIGS. 1-3. The pulse means 18 repeatedly and periodicallysupplies a pulse of negative polarity --Vo to the emitter of transistor10 to forward bias junction 12 and provide a low-impedance path tojunction 14 (FIG. l). The pulse width Al s long enough to allow thetransient condition to cease and permit a steady-state condition. Withthe sum of the voltage drops around a closed loop being zero, it can beseen that the voltage drop across the reverse-biased junction 14 plusthe drop across resistor 22 must be equal to the voltage --Vo (assumingthe drop across the forward-biased emitterbase junction to benegligible). During the steady-state condition, the current throughresistor 22 will be very small and substantially all of the pulsevoltage will appear across the base-collector junction 14, thus chargingthe junction capacitance CBC to a voltage of approximately -Vo as shownin FIG. 2. Thus, the negative polarity pulse results in the base oftransistor 10 having a negative potential with respect to the collector,thereby reverse biasing junction 14 to charge the space-chargecapacitance 0f that junction. At the end of the pulse time At, with thecapacitance CBC charged to voltage V0, the voltage at input electrode tothe emitter goes to zero as does the voltage at the collector electrode.This results in the voltages across both the base-collector junction 14and the emitterbase junction 12 being -Vo, assuming junction 12 has abreakdown greater than -Vo. Thus, junctions 12 and 14 are both in areverse bias condition with the junction 12 providing a very highimpedance and essentially an open circuit to the charge stored injunction 14. The operation of junction 12 very closely approaches thatof an ideal switch. During the period which both junctions 12 'and 14are reverse biased (i.e., the scan time To), the voltage on junction 14will decay, due to the generation-recombination current in thespace-charge region, and also due to any optically generated carriersconstituting a photocurrent which reach the space-charge region ofjunction 14. These two currents tend to discharge the capacitance CBC ofjunction 14 which initially had a charge (CBC) (-Vo) on it. Generally,the photogenerated current is much greater than thegeneration-recombination current. Thus, the charge lost from thespace-charge capacitance of junction 14 during the scan time To isdirectly proportional to the integral of the incident light over theperiod T0. At a subsequent time another pulse having a voltage of -Vo isapplied to the emitter of transistor 10 and a displacement current flowsto replenish the charge lost from junction 14 due to the dischargingcurrents during the scan time To. This displacement current is amajority carrier current to the base, hence the emitter injects minoritycarriers to neutralize the space charge in the base region. This isnormal transistor action and the ratio of charge that flows throughresistor 22 to charge supplied to capacitance CBC is (-j-l) where is thestandard common emitter current gain of the transistor.

It may be helpful to the understanding of the invention to consider theoperation of the device with reference to a number of the physicalrelations that describe its operation. With a negative voltage appliedto terminals 1-1 as shown in FIG. 2, a forward bias will be placed onemitter-base junction 12. Since the voltage on junction 12 cannot changeinstantaneously because of the capacitance CBC, the emitter will beginto inject a current Ie. In this condition, the emitter-base junction hasa high conductance and the current which flows in collector 13 will beale where a is the fraction of injected emitter current which iscollected by the collector. Typically, a may have a value in the rangeof 0 to .9999. The base current is (1- )Ie and must equal thecapacitance displacement current dVCB dt This results in both emitterand collector current depending on the rate of change of thecollector-base voltage. The current that ows through resistor 22 is thesum of the collector current and a base current and is equal to thecurrent IE.

In a time At, a charge equal to the time integral of (1- t)le from time0 to Al is put on the base-collector capacitance CBC and the chargethrough the load resistor 22 is the time integral of Ie from 0 to At.The ratio of charge through the load to charge placed on thebasecollector capacitance CBC turns out to be (-j-l). At the terminationof the negative voltage, the base-collector capacitance CBC is chargedand the base-collector junction is isolated from the rest of the circuitas a result of the low leakage currents permitted by the emitter-basejunction.

So far we have discussed only how to charge the base- CBC collectorjunction and then isolate this charged junction. The charge which hasbeen placed on this junction will decay at a rate that is proportionalto the level of incident illumination. For zero illumination, it willtake in the order of seconds for this charge to decay to half itsinitial value. Under such circumstances (zero illumination), the decaytime is governed by the relationship that generationrecombinationcurrent equals capacitive-displacement current. That is where C=junctioncapacitance d V/ dt=rate of charge of voltageIgr:generation-recombination current Since both capacitance andgeneration-recombination current depend directly on junction area, thearea cancels out of the equation and the rate of change of voltage isindependent of area. Under illumination, the photogenerated current addsto the generation-recombination current to remove more charge per unittime. Since the photogenerated current depends directly on theillumination level, the amount of charge removed per unit time alsodepends directly on illumination level as long as the photogeneratedcurrent is large compared to the generation-recombination current.During the scan time To, a quantity of charge will be removed from thebase-collector capacitance-the magnitude of which is equal to the timeintegral of photogenerated current integrated over the scan time To.When the base-collector junction is recharged, a quantity of chargeequal to (,B-I-l) times the above quantity will ow through resistor 22.Hence, a signal gain is obtained.

In summary, the photosensor described above with reference to FIGS. 1-3provides a low-impedance path through junction 12 for charging thespace-charge capacitance of junction 14. It provides a veryhigh-impedance path approaching an ideal switch by the reverse biasingof junction 12 during the scan time To. The use of transistor 1t)further provides a gain beyond that normally provided in the storagemode operation, thereby improving the signal-to-noise ratio. All of thisis accomplished with a minimum of complexity and in a manner consistentwith present processing technology. The described photosensor device maybe fabricated in the form of integrated circuit arrays.

What is claimed is:

1. A photosensor device for sensing radiant energy comprising:

a transistor connected to operate in the storage mode;

and,

an energizing means coupled to said transistor for periodically chargingthe space-charge capacitance of one junction of said transistor, thecharged capacitance remaining after the removal of said energizing meansto prevent leakage currents from flowing from said one junction throughthe other junction during the intervening periods when said one junctionis not being charged.

2. The structure recited in claim 1 including an output means formanifesting an effect representative of the charge supplied said onejunction during the charging of said one junction.

3. The structure recited in claim 1 wherein said transistor is a planardouble-diffused transistor.

4. The structure recited in claim 3 wherein the area of said onejunction is at least twice that of the other junction.

5. The structure recited in claim 1 wherein said one junction has agiven DC reverse-bias impedance and said other junction has a DCreverse-bias impedance at least an order of magnitude greater than saidgiven impedance.

6. A photosensor device for sensing radiant energy comprising:

a transistor connected to operate in the storage mode having a rst p-njunction and a second p-n junction;

a means coupled to said transistor for repeatedly supplying a pulse tosaid transistor to forward bias said tirst junction and to reverse biasthe second junction;

a network for reverse biasing said rst junction and for maintaining saidsecond junction in a reverse-biased state when said pulse is notsupplied by the pulse means; and

output means for manifesting an effect representative of the chargesupplied to said transistor during the repeated supply of pulsesthereto.

7. A method for sensing radiant energy comprising:

energizing a transistor connected to operate in the storage mode toforward bias one junction and reverse bias another junction to chargethe space-charge capacitance of the reverse-biased junction;

discontinuing said energization to enable said one junction to becomereverse-biased; and

rte-energizing said transistor to again forward bias said one junctionand to recharge the space-charge capacitance of the other junction, saidcharge required to recharge said other junction being representative ofthe radiant energy impinging on the transistor.

8. The method for sensing radiant energy recited in claim 7 whereindiscontinuing said energization maintains said other junction in areverse-biased condition.

References Cited UNITED STATES PATENTS 3,005,107 10/ 1961 Weinstein317-23 5.27 3,348,074 10/ 1967 Diemer 317-235.27 3,378,688 4/1968Kabel1.

RALPH G. NILSON, Primary Examiner.

C. LEEDOM, Assistant Examiner.

